Today's advanced wireless packet data CDMA systems, such as High Speed Data Packet Access (HSDPA) systems, measure the carrier-to-interference ratio (CIR) at a mobile terminal, and based on this measurement, send channel quality indicator (CQI) information to the base station. The CQI information, as defined using the CIR measurement for CDMA systems, is a reliable indication of the error rate expected at the mobile terminal. CQI information is continuously reported back to the base station because the transmission channel conditions change as the mobile terminal and objects around the mobile terminal move. Therefore, the CQI information reporting occurs on a regular basis, continuously updating the base station of the changing channel conditions experienced by the mobile terminal. The base station receives CQI information for each active mobile terminal that is attempting to share a common transmission channel.
The base station uses the CQI information for two primary purposes. The first purpose is to select from various levels of modulation and coding to use for transmissions to the mobile terminal in an effort to maximize the throughput to that particular mobile terminal or for the system in general. The second purpose is to assist in scheduling when and how much information should be sent to the mobile terminal and other mobile terminals competing for channel resources. Depending on the scheduling criteria, the base station may choose to send more data to those mobile terminals experiencing good channel conditions to maximize the system's overall throughput. For example, the scheduling criteria may dictate sending information only to those mobile terminals having channel conditions sufficient to support the highest data rate or rates.
In orthogonal frequency division multiplexing (OFDM) systems, the CIR may also be used as a CQI in the same fashion as it is used for CDMA. When one plots the average error rate at the mobile terminal as a function of the CIR, the result is a monotonically decreasing function. However, OFDM and CDMA communications differ in some fundamental ways. One particular difference is the mechanism by which frequency diversity is achieved in each system. Due to this difference, CIR alone is not the most reliable indication of the error rate to be expected at the mobile terminal. As illustrated in FIG. 1, for a given CIR, there is a fairly large variance in the error rate. A large variance in the error rate for a given CIR implies that there are other factors that influence the error rate, not just the CIR. In contrast, CDMA systems have a much smaller variance in the error rate at a particular CIR at the output of a receiver, and as such, the CIR alone has proven to be a good CQI. Since CIR alone is not sufficient for efficient OFDM systems, there is a need for an improved CQI for OFDM systems that takes into account the frequency response of the channel.